There are a variety of device applications that require the generation of electrical pulses. Electrical pulses are typically generated by electrical circuits called “pulse generators”. Pulse generators are used, for example, to create the timing signals necessary to coordinate the logic operations of a microprocessor. Pulse generators generally fall into two categories: clock based and delay based.
Clock-based pulse generators are generally capable of generating arbitrarily long pulses. However, the minimum time interval between pulses is limited by the clock period.
Delay-based pulse generators rely on the time delay incurred by passing signals through a device or set of devices. The time delay is based on the transition time or difference in transition time of signals. The disadvantages of this type of pulse generator include that the maximum delay that can be obtained is limited, and the timing jitter increases with the delay or pulse width.
There are some device applications that would benefit from a pulse generator that has the advantages of each of the above-mentioned pulse generators while minimizing their respective disadvantages.
One such application is quantum key distribution (QKD). The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175–179 (IEEE, New York, 1984). Specific QKD systems are described in publications by C. H. Bennett et al entitled “Experimental Quantum Cryptography,” and C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992), as well as in U.S. Pat. No. 5,307,410 to Bennett (the '410 patent). The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27–33.
Generally, in QKD weak light pulses (e.g., less than one photon per pulse on average according to Poissonian statistics) need to be emitted and detected with high precision. This requires generating precise synchronization and gating pulses for the system. The above-mentioned shortcomings of the prior art pulse generators make them inefficient and impractical to use for a commercially viable QKD system. For example, one pulse generation mode for generating long (e.g., on the order of microseconds) pulses is required for setting up and calibrating the system, while another pulse generation mode is required for generating pulses on the order of 500 ps when operating the system. While two different types of pulse generators could be used to generate the two types of pulses, this option is expensive and adds complexity to the system.